Field of the Invention
The present invention relates to an image forming (objective) optical system which can be used in an endoscope apparatus used in a medical field or an industrial field.
Description of the Related Art
Endoscope is an apparatus Which is used widely in the medical field and the industrial field. In the medical field, images of various sites inside a body cavity are achieved by an endoscope inserted inside the body cavity. Diagnosis of a site observed is carried out by using these images. In such manner, endoscopes have been used for observation and diagnosis of various sites inside the body cavity.
In an objective optical system for endoscope, an optical member such as a prism may be disposed in an optical path. Therefore, in the objective optical system for endoscope, sometimes a long back focus is necessary. Objective optical systems having such long back focus have been proposed in Japanese Patent Publication No. 4919419 and Japanese Patent Publication No. 4675348.
Moreover, in endoscopes, increasing the number of pixels of an image pickup element has been progressing in recent years. It is necessary to make an optical spot formed by an optical system all to deal with the increase in the number of pixels. However, if increasing the number of pixels is not dealt with, a quality of an image that is picked up is degraded due to diffraction.
To prevent the degradation of image quality, it is necessary to make an F-number of the objective optical system small. Consequently, in objective optical systems in recent years, a depth of field tends to be narrow. As a method of securing the depth of field of a wide range according to such image pickup element, a method of imparting a focusing function to the optical system is available. Optical systems having the focusing function have been proposed in Japanese Patent Publication No. 4819969, Japanese Patent Application Laid-open Publication No. 2012-37768, and Japanese Patent Publication No. 5607278.